The vascular system is subject to various disorders, among the more serious being occlusive diseases and aneurysms. Occlusive diseases refer to the closing of vessels such as the iliac, carotid, biliary and coronary arteries. Aneurysms are defined as a pathologic dilation of a vessel and can affect the aortic, thoracic, renal and iliac arteries among others.
Effective treatment of many occlusive diseases and aneurysms is possible through the use of an endovascular stent graft. The stent graft comprises a tubular structure (the “graft”) which effects a connection between portions of a vascular vessel. The graft is typically flexible and preferably formed of biocompatible filaments or yarns interlaced by weaving, knitting or braiding to form a tube or sleeve. The stent is positioned circumferentially around and supports the graft and is preferably formed from elastic, resilient materials such as nitinol, titanium or stainless steel which have a high yield stress and are also compatible for implantation within the human body.
A particular example of the use of an endovascular stent graft to treat an aortic aneurysm is described below, it being understood that the example is for illustrative purposes only and in no way limits the invention disclosed herein to the treatment of any particular vascular disorder, as will be appreciated by those of skill in the art. It is further recognized that the invention is also applicable in the treatment of other occlusive diseases or aneurysms, such as neurological, esophageal and bronchial aneurysms.
An aneurysm, as noted above, is a pathologic dilation of a segment of a blood vessel which constitutes a weakened portion of the vessel. In a fusiform aneurysm 10, such as can occur in the abdominal aorta 12 as seen in FIG. 1, the entire circumference of the vessel is dilated and weakened. The majority of these aortic aneurysms are located in the distal abdominal aorta between the renal arteries 14 and the bifurcation point 16 where the abdominal aorta splits into the common iliac arteries 18.
Abdominal aortic aneurysms are the 13th largest cause of death in the United States. Such aortic aneurysms constitute a serious condition, as an acute rupture of the aneurysm is fatal unless an emergency operation is performed. However, even when such operations are performed in time, the mortality rate is still greater than 50%.
Modern methods of treatment for aortic aneurysms focus on providing an endovascular stent graft which is inserted within the artery at the aneurysm by means of a catheter. As seen in FIG. 1, stent graft 20 comprises a tubular graft 22, which could be woven, knitted or braided, graft 22 having one end 24 which is attached to the inner surface of the artery above the aneurysm 10. The opposite end 26 of the stent graft is split into two tubular grafts 26a and 26b which branch into and are attached to the inside surfaces of the iliac arteries 18 below the aneurysm 10. The stent graft replaces the abdominal aorta in the region of the aneurysm 10, relieving the pressure on the arterial wall and avoiding a potentially fatal rupture.
Stent graft 20 further comprises stents 28 and 30 in the form of continuous wire springs positioned in the ends 24 and 26a and 26b of the graft. Also positioned at the ends are hooks 32, arranged circumferentially around the graft with hook ends 34 facing outwardly.
The stents 28 and 30 are each formed into a plurality of crests and troughs circumferentially of the graft and normally bias the graft ends into a substantially circular, open configuration when the graft is positioned within the artery. The stents are elastically collapsible to a small diameter, however, so as to enable the graft to be advanced through the bore of a catheter which fits within the artery.
To implant the stent graft, the catheter is inserted into the aorta from one of the iliac arteries, the catheter tip being positioned within the artery above the aneurysm. The stent graft is advanced through the catheter, and when the end 24 emerges from the catheter, stent 28 expands radially outwardly under the biasing spring forces of the stent, opening the graft and filling the artery diameter. Ends 34 of hooks 32 positioned at end 24 engage and grip the inner surface of the artery, fixing the stent graft into position. The catheter is then withdrawn as the stent graft is released from the catheter. When the ends 26a and 26b emerge, the respective stents 30 expand radially within the artery, opening the stent graft 20 at ends 26a and 26b. The ends are positioned into a respective iliac artery where hooks 32 engage and grip the interior surface of the iliac arteries, thereby fixing the stent graft 20 into position within the aneurysm 10.
Use of the endovascular stent graft has proved effective at treating fusiform aneurysms, especially aortic aneurysms as described above. However, the manufacture and use of stent grafts is troublesome and expensive for several reasons.
Sutures are a Disadvantage
Normally, the stents 28 and 30 and the hooks 32 are sutured to the graft 22. Suturing is also used to attach other items, such as radiopaque markers to the graft. Suturing is necessarily a hand operation given the small size, delicacy of the components and the precision required to produce a usable stent graft. Hand suturing is done by skilled workers, and each piece takes a relatively long time to complete, resulting in low production rates and high production costs.
Furthermore, sutures can become untied during the operation or after the stent graft is implanted. If the sutures fail during the operation and the stent or hooks separate from the graft (even partially), then all of the pieces have to be retrieved and removed from the artery before another attempt to implant a stent graft can be made.
The consequences of a failure after an operation could, of course, be fatal. For example, if the end 24 of the graft separates from the stent 28 or hooks 32 due to a suture failure and the graft end 24 fails to seal within the artery or collapses into the aneurysm, the stent graft will no longer reinforce the artery, the aneurysm will be subjected to the blood pressure and acute rupture could ensue. The free graft end could also fold and partially block the artery, again with potentially fatal results.
The use of sutures also adds bulk to the stent graft. This is undesirable because it limits the minimum size of the catheter usable to implant the stent graft. The stent graft must fit within and advance along the bore of the catheter during the operation, and the bulkier the stent graft the larger a catheter is required, thereby limiting the size of artery which can be treated by the implantation of a stent graft.
Sutures also tend to snag on the walls or tip of the catheter, thus, making it difficult to smoothly advance the stent graft along the bore or readily release it from the catheter, potentially complicating the operation.
Finally, because sutures are expensive and time consuming to use, tend to increase the bulk of the stent graft and tend to snag the walls of the catheter, they are used sparingly to attach the stent and hooks to the graft. Fewer attachment points means greater potential for failure at some point during or after the operation. Clearly, there is a need for an improved stent graft wherein the stents and hooks are attached to the graft in a way which avoids the disadvantages of sutures.
Delivery and Positioning of the Stent Graft Needs Improvement
As described above, stent grafts are delivered percutaneously by means of a catheter inserted within an artery. The catheter must traverse a significant length of the artery before the catheter tip is positioned so that the stent graft may be released and expanded to attach itself to the artery wall. To accurately and easily position the catheter tip, the catheter must be flexible so as to follow the twists and turns of the artery. Stent grafts positioned within the catheter tend to stiffen it and reduce its ability to bend and easily traverse the artery for accurate positioning of the tip. The longer and more bulky the stent grafts are the more they stiffen the catheter, thereby increasing any disadvantage. Furthermore, stent grafts generally require a sheath within the catheter, thereby further increasing bulk and thus stiffness of the catheter.
Accuracy of placement is of particular importance for treatment of an aortic aneurysm, for example, so that the stent graft is attached to healthy tissue above the aneurysm but below the branch point of the renal arteries. Accurate placement insures that the stent graft will form a strong sealing attachment to healthy tissue without blocking blood flow through neighboring arterial branches.
Different Sizes of Stent Graft are Required
To effectively treat an aneurysm, the stent graft must be sized appropriately to the patient, i.e., the diameters of the attachment points must be such that they form an effective seal with the artery, and the lengths of the various branches must bridge the diseased region without being too short or having significant excess length. The diameter of the stent must match the artery so that there is smooth and substantially continuous contact between the graft and the artery wall around the circumference. Too small a stent diameter will cause one or more folds to occur in the artery, leading to an incomplete seal and leakage into the aneurysm. Too large a stent diameter will result in a fold in the graft, also causing a leak as well as an obstruction within the artery that impedes the flow of blood and which may become a point where clots form and break off to later lodge in the artery and form an embolism. If the length of a branch is insufficient, then the branch may not bridge the diseased region of the artery, placing additional stress on an already weakened artery wall. If the lengths are too great, then the branches may kink and impede blood flow.
Assurance that the proper size stent graft is available requires either that each stent graft be custom made for each patient, or that a large number of stent grafts covering a broad spectrum of sizes be available. The impracticality of both solutions leads to significant expense in the treatment of vascular disorders.
Clearly, there is a need for improved stent grafts for the treatment of vascular disorders.